1. Field of the Invention
The invention relates to a polymer coated metal surface in which at least one polymer layer is covalently bonded to the activated metal surface. The polymer coating may contain one or more biologically active agents. The polymer coated metal can be used in an implantable medical device such as a stent. The invention further relates to methods of coating metals surfaces and preparing medical devices.
2. Description of the Related Art
Many surgical interventions require the placement of a medical device into the body. While necessary and beneficial for treating a variety of medical conditions, the placement of metal or polymeric devices in the body can give rise to numerous complications. Some of these complications include increased risk of infection, initiation of a foreign body response resulting in inflammation and fibrous encapsulation, and/or initiation of a wound healing response resulting in hyperplasia and/or restenosis. These and other possible complications must be dealt with when introducing a metal or polymeric device into the body.
One approach to reducing the potential harmful effects of such an introduction has been to attempt to improve the biocompatibility of the device. While there are several methods available to improve the biocompatibility of devices, one method which has met with limited success is to provide the device with the ability to deliver biologically active agents to the vicinity of the implant. By so doing, some of the harmful effects that can be associated with the implantation of medical devices can be diminished. For example, antibiotics can be released from the device to minimize the possibility of infection, and anti-proliferative drugs can be released to inhibit hyperplasia. Another benefit to the local release of biologically active agents is the avoidance of toxic concentrations of drugs which are sometimes necessary when given systemically to achieve therapeutic concentrations at the site where they are needed.
Those skilled in the art of medical devices have been challenged to meet the several stringent criteria for implantable medical devices. Some of these challenges are: 1) the requirement, in some instances, for long term (days, weeks, or months) release of biologically active agents; 2) the need for a biocompatible, non-inflammatory surface on the device; 3) the need for significant durability, particularly with devices that undergo flexion and/or expansion when being implanted or used in the body; 4) concerns regarding processability, to enable the device to be manufactured in an economically viable and reproducible manner; and 5) the requirement that the finished device be capable of being sterilized using conventional methods.
Several implantable medical devices capable of delivering medicinal agents have been described. Several patents are directed to devices utilizing biodegradable or bioresorbable polymers as drug containing and releasing coatings, including Tang et al, U.S. Pat. No. 4,916,193 and MacGregor, U.S. Pat. No. 4,994,071. Other patents are directed to the formation of a drug containing hydrogel on the surface of an implantable medical device, these include Amiden et al, U.S. Pat. No. 5,221,698 and Sahatijian, U.S. Pat. No. 5,304,121. Still other patents describe methods for preparing coated intravascular stents via application of polymer solutions containing dispersed therapeutic material to the stent surface followed by evaporation of the solvent. This method is described in Berg et al, U.S. Pat. No. 5,464,650.
A number of approaches have been used to try to overcome the challenges listed above. The below are examples of these approaches.
McPherson, et. al., U.S. Pat. No. 6,013,855, describes methods for grafting hydrophilic polymers onto metal surfaces. This method included exposing the device surface to a silane coupling agent and causing the agent to be covalently bound to the device surface. The bonded silane layer was then exposed to a hydrophilic polymer such that the hydrophilic polymer became covalently bound to the silane layer.
Pinchuck, U.S. Pat. No. 5,053,048, describes curing a silane compound or compounds onto a surface to form a hydrophilic matrix. An antithrombogenic agent was then coupled to the amine group on the aminosilane three-dimensional matrix to provide a thromboresistant coating to the surface.
Lee, et. al., U.S. Pat. No. 6,335,340, describes methods for coating oxide surfaces and coatings that rendered such surfaces hydrophilic. A functional group (Z) such as SiCl3 was associated with the surface. A tether of a hydrophobic covalent attachment, typically of approximately 5 to 20 bonds in length, was formed with Z. A biopolymer-resistant domain was then adhered to the tether to form the hydrophilic surface.
Hostettler, et. al., U.S. Pat. No. 6,265,016, describes chemically treating metal surfaces to affix amine-containing groups. A “tie coat” of a hydrophilic polyurethane was then covalently attached to the amine groups to form a slippery, hydrophilic polyurethane hydrogel.
Kamath, et. al., U.S. Pat. No. 6,335,029, describes applying at least one composite layer of a biologically active agent and a polymer to a base material by physical or covalent methods. At least one barrier layer was positioned over and applied to the composite layer by a low energy plasma polymerization process.
Shah, et. al., U.S. Pat. No. 6,248,127, describes coatings for medical devices in which a silane coating is adhered on the surface of the substrate and a film containing a heparin-biopolymer complex is created on the surface by covalent linkages.
However, there remain significant problems to overcome in order to provide a durable implantable medical device capable of delivering a therapeutically effective amount of a biologically active agent for an extended period of time. This is particularly true when the coating composition must be kept on the device in the course of flexion and/or expansion of the device during implantation or use. It is desirable to have a facile and easily processable method of controlling the rate of biologically active agent release from the surface of the device.
Although a variety of polymers have previously been described for use as drug release coatings, only a small number possess the physical characteristics that would render them useful for implantable medical devices which undergo flexion and/or expansion upon implantation. Many polymers which demonstrate good drug release characteristics, when used alone as drug delivery vehicles, provide coatings that are too brittle to be used on devices which undergo flexion and/or expansion. Other polymers can provide an inflammatory response when implanted. These or other polymers demonstrate good drug release characteristics for one drug but very poor characteristics for another.
In many respects, the success of a polymer coating depends on the nature of the contact between at least the polymer layer adjacent to the metal surface and the underlying metal surface. In particular, if the polymer cracks or peels away from the metal surface, the polymer and any biologically active agent contained therein may decrease in performance. If the polymer layer is designed to contain a biologically active agent to be released, the resulting polymer/biologically active agent composite may be prone to dilation, swelling, degradation, and/or volume changes because of interactions of the incorporated compound with aqueous environments of the body. Also, following the penetration of water into the polymer layer, dissolution of the compound and its subsequent release, may change the structure and porosity of the composite. In addition, due to penetration of water following drug dissolution, the polymer layer could be exposed to a mechanical stress due to osmotic forces. These effects may result in detachment of the polymer layer and its peeling from the metal surface. Further, the changes in the geometry of the polymer layer and the available surface area are potential sources of unpredictability of the release rate for the incorporated compounds. Due to a combination of these factors, the performance of the system decreases.
Accordingly, there is a persistent need for an improved polymer coating of metallic implants that provides a stable, biocompatible and low-profile polymer coating that, at the same time, provides a long-term release of biologically active agents for periods extending to weeks or months. Thus, there is a need for a method for securing the highly reproducible deposition of the polymer coating layer on the article surface. In many instances the polymer layer has to be thin enough so that it does not restrict the flexibility and adaptation of the metal device. Also, the polymer layer must resist damage due to device handling or deformation.